Hitherto, for example, in an apparatus in which at least a part of the displaying functions of its display unit is operated by photoelectrically generated power, a light-receiving surface of a solar battery is disposed on a part of or the entire displaying surface formed in the display unit, and electric power is generated from light incident on the light-receiving surface. Among such apparatuses, a typical example is a watch in which a light-receiving surface of a solar battery is disposed instead of a part of or the entire dial. In such a watch, since a light-receiving surface of a solar battery is placed in the position which should be a surface of its dial, another light-receiving surface in a position other than the displaying surface is not required, the space for the light-receiving surface can be saved, and therefore, even a small watch can contain a solar battery which has a power-generating ability sufficiently coping with the electric power demanded from its clocking function.
In early solar-battery-mounted watches, however, since the light-receiving surface of the solar-battery was exposed to the displaying surface where a dial should originally be disposed, the appearance of the displaying surface was unsatisfactory, and achieving a fancy design was difficult. FIG. 12 is a plan view of an example of a solar battery mounted in such a watch. On the surface of the solar battery 72, a light-receiving surface 72a is formed which has a planar pattern comprising four partitions 72a-1, 72a-2, 72a-3 and 72a-4 separated by gapping portions 72a-0. Here, the gapping portions 72a-0 ordinarily have a light color tone, and partitions 72a-1, 72a-2, 72a-3 and 72a-4 ordinarily have a dark color tone. Accordingly, such a separated pattern gives a markedly strange impression for the displaying surface, and is disadvantageous in view of design. In particular, although characters and decorative parts are desired to be added onto the dial of a watch by printing, sticking or other methods, such characters and decorative parts could rarely be directly formed on the light-receiving surface 72a of the solar battery.
Upon this, hitherto, as shown in FIG. 13, a technique was offered in which a translucent coating layer 73 is formed on a light-receiving surface 72a, and the displaying surface can thereby possess an appearance with a predetermined color tone. Such a technique is disclosed, for example, in Japanese Unexamined Utility Model Publication No. 5-93057 and others. Inside the body case 70 (trunk) of a watch, a dial holding ring 71 and a dial ring 74 are fixed, and a solar battery 72 is fixed between the dial holding ring 71 and the dial ring 74. A coating layer 73 is formed on the light-receiving surface 72a of the solar battery 72, and the surface of the coating layer 73 serves as a dial surface of an ordinary watch. Incidentally, a hands/wheels portion 75 protrudes to the front surface through a wheel hole formed at the center of the solar battery 72, and a cover glass 76 fixed to the body case 70 is disposed above the hands/wheels portion 75.
The coating layer 73 is formed as a light-transmissive layer having a predetermined transmittance, or a scattering layer having a predetermined wavelength-selective scattering property. In this case, since a part of light is reflected or scattered in the coating layer 73 and the remaining transmitted light which reaches the light-receiving surface 72a is used for power generation in the solar battery, it is an important point to adjust the optical properties of the coating layer 73 to desired levels. In- such a structure, the light-receiving surface 72a can be protected by the coating layer 73 while securing the power-generating function of the solar battery, and the coating layer 73 can give the displaying surface an appearance with a predetermined color tone, characters, decorative parts and the like.
In the above-described solar-battery-mounted watch, when the light transmittance of the coating layer 73 is high, the dark partitions 72a-1, 72a-2, 72a-3 and 72a-4 of the solar-buttery light-receiving surface 72a having a planar pattern as shown in FIG. 12 can be seen through the layer in the shapes separated by the light gapping portions 72a-0 which are formed between the partitions. As a result, the appearance of the displaying surface is poor. In contrast, when the light-transmittance of the coating layer 73 is low, the amount of light reaching the light-receiving surface 72a of the solar battery is lowered, and the generated electric power is reduced to cause problems in the operation of the watch. Accordingly, the optical properties of the coating layer 73 should be adjusted such that the displaying surface has an appearance with a predetermined color tone and the like while achieving a light transmittance at a degree needed to secure power generation by the solar battery. Ordinarily, the thickness and material of the coating layer 73 should be appropriately selected and-adjusted in order to achieve desired optical properties of the coating layer. It is, however, extremely difficult to select materials satisfying the above requirements from among materials which can be applied onto the light-receiving surface 72a of the solar battery, and therefore, the restriction on the color tone and design cannot sufficiently removed. Further, in this case, since such an additional coating layer 73 must be deposited on the light-receiving surface 72a of the solar battery according to a physical or chemical method, the manufacturing costs are increased. Moreover, if the coating layer 73 formed on the solar battery 72 has a defect or is damaged afterward, the resultant must be discarded together with the expensive solar battery 72.
Meanwhile, Japanese Unexamined Patent Publication No. 7-72266 and others have disclosed some methods in which a transparent or translucent plate (hereinafter referred to as light-transmissive plate) such as a resin plate, a ceramic plate or a marble plate is disposed on the light-receiving surface of a solar battery. According to such a method, the manufacturing cost can be lowered as compared to a method in which a coating layer is directly formed on the light-receiving surface of a solar battery, and desired characters and decorative parts can be provided on the front surface of the light-transmissive plate by printing or the like. In such a case where a light-transmissive plate is disposed on the light-receiving surface, consideration for suitability of the coating layer to the light-receiving surface of the solar battery, which is required in the above-mentioned example, is not especially required. Nevertheless, similar to the above, the light transmittance of the light-transmissive plate must be precisely controlled by selecting the thickness and material in order to improve the appearance of the displaying surface and secure sufficient electric power generation. In this case, if the material for the light-transmissive plate is selected in view of appearance only, the thickness of the plate tends to be excessively thin to make the assembling work difficult, or the light-transmissive plate tends to suffer from deformation such as bending even after being assembled. Furthermore, when the light transmittance of the light-transmissive plate is relatively high, interference fringes are generated due to surface reflection between the light-receiving surface of the solar battery and the light-transmissive plate, and the appearance of the watch is thereby spoiled. In particular, even though the design of the displaying surface is expressly satisfactorily improved by providing a light-transmissive plate, the quality of the product may be doubted due to the presence of interference fringes.
The present invention is directed to solution of the above-described problems, and an object of the invention is to provide a solar-battery-mounted display structure in which the light-receiving surface of the solar battery is disposed on a displaying surface, wherein the appearance of the displaying surface can be improved, the power-generating capacity of the solar battery can be relatively readily secured, and the display structure can be easily manufactured. Further, the present invention provides a display structure which is free from interference-fringe generation by the light-receiving surface of the solar battery and a coating layer or a light-transmissive plate. Moreover, in the present invention, prevention of interference-fringe generation is achieved according to an easy and manufacturing-cost-saving method. In particular, the subjects to which the solar-battery-mounted display structure is applied are solar-battery-mounted electronic apparatuses and solar-battery-mounted watches as the quality of their displaying portions can be improved and their sizes can be reduced.